Continuously operating digesters for cooking comminuted cellulose containing material to paper pulp have been known for a long time and hence also feeding systems for such digesters. The requirements of the feeding system are, among other things, that the cellulose material, hereinafter called chips, should be evenly fed from a low (atmospheric) pressure to a higher pressure, that the chips should be heated at the same time as vapor and gases are removed from the chips and replaced with water or condensate, and that the feeding system should be as inexpensive as possible in terms of investment costs and operating costs.
A conventional feeding system typically includes a chip bin, a chip meter, a low pressure feeder, a steaming vessel, a chip chute and a high pressure feeder. The function of the high pressure feeder is to convey the cellulose material, including some liquid, to a continuous digester or to a pre-impregnation vessel, that operates at a relatively high pressure. Between the high pressure feeder and the impregnation vessel or digester, there is conventionally a top circulation, which comprises a feed line for feeding a mixture of chips and impregnation liquid to the vessel, and a return liquid line for separating the impregnation liquid from the chips. A top separator is arranged at the top of the impregnation vessel or the digester for feeding the chips into the impregnation vessel or digester at the same time as a portion of the impregnation liquid is separated off and pumped with a pump through the return liquid line back to the high pressure feeder. The high pressure feeder is equipped with a rotor with pockets so that one of the pockets is always in a low pressure position and in fluid communication with the chute. At the same time, another pocket is always in a high pressure position and open in connection with the impregnation vessel or digester via the feed line. When a rotor pocket which is, to a degree, filled with chips, is moved into the high pressure position, i.e., in direct connection with the top circulation, the rotor pocket is flushed clean by the liquid from the return liquid line and the suspension of chips and impregnation liquid is then fed into the top of the impregnation vessel or digester via the feed line. The liquid that flows in a circulation loop on the low pressure side of the high pressure feeder, which loop is conventionally equipped with a pump, is at the same time feeding chips from the chute into one of the next pockets of the high pressure feeder so that this pocket is, to a degree, filled with chips. The circulation loop is also equipped with a sand trap and a tubular screen, referred to as an in-line drainer. Furthermore, a level tank is, via a line from the in-line drainer, connected to the return liquid line of the top circulation.
This conventional system was developed a long time ago when the production volumes of continuous pulp mills were perhaps just one tenth of the volume that modern pulp mills produce today. Accordingly, when the conventional feeding system was developed, the machines used were mush smaller. For example, the pump that was disposed in the circulation loop, for recirculating liquid to the chute on the low pressure side of the high pressure feeder, was rather small and hence needed to be protected from chips that might enter the circulation loop together with the liquid that was discharged from the high pressure feeder. To provide this protection, the high pressure feeder was equipped with a screen device, a so called strainer plate, at its outlet leading to the circulation loop. The intention was that the liquid should pass the screen and that the chips should remain in the pocket of the high pressure feeder to give a high filling degree of the pocket. In practice, however, the screen was partially plugged every time a pocket was filled with chips. This resulted in a filling degree that was only about 50-70% of the total filling volume of chips of the pockets. The rest of the filling volume was filled with liquid. This should be compared with the filling efficiency in the chute, which theoretically should be able to be reached in the high pressure feeder. The theoretical filling efficiency in the chute is about 80-85% of the total filling volume. When the rotor turns, the strainer plate is often scraped free from the chips, but the problem re-occurs when the next pocket comes in the same position. The partial filling degree results in an inefficient operation and in that the high pressure feeder must be operated at a relatively high rotary speed that, in turn, leads to a substantial wear on the equipment.
In addition, the operational volume should be relatively large in order to accommodate the high pressure feeder that must be positioned at a relatively high level due to the required suction pipe for the pump on the low pressure liquid side of the high pressure feeder. Also, the operational volume must be large enough to accommodate the sand trap, in-line drainer and level tank. These requirements increase the investment and operating costs.
Surprisingly, it has now been found that a well functioning feeding system can be provided that operates without a screening device in the high pressure feeder and also without an in-line drainer and a level tank. The feeding system according to the present invention further enables the pockets of the high pressure feeder to be filled to a theoretical maximum degree, i.e., to the same degree as the filling efficiency in the chute.
As mentioned above, the machines in modern high production pulp mills are much larger than they were at the time when the conventional feeding systems were developed. In particular, the pump in the circulation loop, for recirculating the liquid to the chute on the low pressure side of the high pressure feeder, is much larger today and is capable of handling a large amount of chips in the liquid flow, since the size of the chips themselves is not much different from what it has always been. An important feature of the present invention is that the screening device that prevented the chips from entering the circulation loop can be excluded, which results in several advantages.
Firstly, when a pocket of the high pressure feeder is moved towards the low pressure position, the pocket is full with liquid from the return liquid flow from the digester or pre-impregnation vessel, hereinafter called the treatment vessel. When the pocket reaches the low pressure position, the liquid is displaced with the mixture of chips and liquid that is present in the chute so that the same filling efficiency as in the chute can be achieved. The filling efficiency in the chute is normally about 80-85% of the total volume since some excess liquid is required for the chip column to be able to move down into the high pressure feeder.
Secondly, the required operational level of the high pressure feeder can be lowered. This is a consequence of the fact that there is essentially no suction pipe needed for the pump in the recirculation flow of the circulation loop since there is no pressure drop across a screening device. Also, the conventional level tank and its in-line drainer can be excluded from the system, which results in reduced investment and operating costs as well as in a smaller building volume. One reason for the possibility of excluding the level tank and the in-line drainer is that in the feeding system of the present invention, there is always a liquid communication between the pumps on the liquid side of the high pressure feeder and the chute. This means that a chute liquid level control valve can be placed in connection with one of these pumps for regulating the liquid level in the chute. In the conventional system, there is no such liquid communication when the screening device is plugged and the chute liquid level control valve must be connected with a level tank, normally between the in-line drainer and the level tank.
Another advantage is that the rotary speed of the pockets in the high pressure feeder of the present invention can be increased to a speed that is up to twice as high as in a conventional feeding system. Thereby, there is a major increase in the capacity of the high pressure feeder.
An additional advantage of the feeding system of the present invention is that the pump, hereinafter called the first pump, in the recirculation conduit can be coupled in series with a second pump that pumps liquid from the low pressure recirculation conduit to the high pressure return line from the treatment vessel. This means that the pump head of the first pump can be added to the pump head of the second pump so that the second pump may be one standard pump instead of, as in the conventional system, two standard pumps or one high pressure pump with several impellers. This results in a major reduction in investment and operating costs.
According to another aspect of the present invention, the recirculation flow, i.e., the volumetric flow that is discharged from the high pressure feeder on its low pressure side, is related by a factor 0.8-1.5 to the volumetric flow which is handled by the high pressure feeder. The volumetric flow through the high pressure feeder on the low-pressure side can be calculated as the volume of the pockets in the high pressure feeder multiplied with the rotary speed of the high pressure feeder and by a factor two (since the pockets are filled twice in each complete rotation). Another way of calculating the volumetric flow is by dividing the incoming chip flow (as measured in a chip mater) by a factor 0.5-0.9. This factor corresponds to the degree of chip filling of the high pressure feeder.
According to yet another aspect of the invention, a sand trap is installed in the return liquid line. This location has the benefit, as compared to the conventional location in the circulation loop, of providing a steady flow without any essential fluctuations in its Velocity. The sand trap consists of a cyclone which has to be optimized for a certain flow velocity and operates better in the relatively steady return liquid flow from the treatment vessel. In the alternative, the sand trap may be installed in the chute. A further advantage of moving the sand trap from its conventional location is that the sand is not circulated in the recirculation flow on the low pressure side of the high pressure feeder so that the wearing effect on the equipment is avoided. These alternative placements of the sand trap can of course also be included in other types of feeding systems.